1. Field of the Invention
The invention relates to a data transmission system for transmitting data between a rotating part and a stationary part, in particular between the rotating part and the stationary part of a computer tomograph, and also to a computer tomograph having a corresponding transmission system.
2. Description of the Relevant Art
Rotatable units such as radar devices or computer tomographs often require transmitting electrical signals or energy in either direction between a rotating and a stationary part. For this, usually a conductor structure for conducting electrical signals to be transmitted is provided in a primary unit, and a suitable tap or probe in a secondary unit. These transmission systems are known as rotary joints and are categorized into contacting and non-contacting rotary joints. The contacting rotary joints mostly have a comparatively solid conductor in the primary unit and a brush like a wire or a carbon brush in the secondary unit. The brush is in electrical and mechanical contact with the conductor. The contacting rotary joints are also known as sliprings. Non-contacting rotary joints generally have a conductor designed as a transmission line which may be a printed circuit board in the primary unit and which is generally designed to generate low far-field radiation while conducting the signal to be transmitted. In the secondary unit there is a comparatively small probe which capacitively picks up near field signals from the conductor of the primary unit.
Non-contacting rotary joints are often used in computer tomography (CT) scanners. The transmission line being part of the rotating part of the gantry is often attached to an outer surface of said rotating part. In most cases it has a diameter of approximately 1.5 meters. This results in an overall length of the transmission line of about 4.5 meters. Up to date, CT scanners have to transmit data at a rate of more than 5 GB per second, up to 30 GB per second. In the future, even higher data rates are expected. It is obvious that the transmission line has to carry signals at high frequency in the Gigahertz frequency range. The transmission line itself is designed similar to a strip line and may have a specific pattern for filtering signals. It must be highly symmetric to avoid far field radiation which leads to enhanced electromagnetic radiation of the scanner, which is not desired. Furthermore, the transmission line must have low losses to lead the high frequency signals without significant attenuation to maintain a good signal/noise ratio over the full length of the transmission line and therefore independently of the rotational position of the CT scanner gantry.
U.S. Pat. No. 5,579,357 discloses a transmission line which is split into two equal parts. Each part receives the signals to be transmitted by a phase splitter at one end and is terminated at the other end. Here, still each section of the transmission line has a length of more than 2 meters which is still difficult to manufacture.
Such transmission lines may be manufactured by using enhanced glass fiber reinforced epoxy printed circuit boards. Modern printed circuit board technologies using substrates based on PTFE (polytetrafluorethylene) or ceramic materials are only available for small RF printed circuit boards and cannot be used here.